$\dfrac{3}{6} + \dfrac{9}{4} = {?}$
$ = {\dfrac{3 \times 2}{6 \times 2}} + {\dfrac{9 \times 3}{4 \times 3}} $ $ = {\dfrac{6}{12}} + {\dfrac{27}{12}} $ $ = \dfrac{{6} + {27}}{12} $ $ = \dfrac{33}{12}$